Method for the regeneration of humidity-laden process air and arrangement for carrying out said method

ABSTRACT

A method for regeneration of humidity-laden drying cartridges is disclosed, in which atmospheric air is heated to ca. 220 to 300° C. and introduced into the drying cartridge ( 24   a   , 24   b ) for regeneration. The subsequent cooling of the drying cartridge ( 24   a   , 24   b ) is achieved by means of a partial stream of air diverted from the dried process air. An arrangement suitable for carrying out said method is also disclosed.

[0001] The invention relates to a method for regenerating moisture-ladenprocess air of the introductory portion of claim 1, as well as to anarrangement of claim 5 for carrying out the method.

[0002] In numerous manufacturing processes, especially when processingplastics, the starting materials and/or the intermediate products mustbe dried before they are processed further. At the same time, theprocess air, used for the drying process, accumulates moisture, whichwas withdrawn from the starting materials and/or the intermediateproducts. Basically, the so-resulting moisture-laden warm process aircould be discharged untreated to the environment and replaced by freshair. However, such a solution is incompatible with conserving energy andtherefore also excluded for reasons of costs.

[0003] When plastic granulates are processed, it is necessary, as isalready mentioned above, to dry the plastic granulates before they areprocessed or processed further. This is done by means of warm, dry air(process air), which flows through the plastic granulates in acontainer, provided for this purpose, takes up the moisture to beginwith and then subsequently must be freed once again from the moisturetaken up.

[0004] For this purpose, the process air, in which the moisture hasaccumulated, is subjected to a regeneration process for removing thismoisture from the drying cartridges filled with molecular sieves, bywhich the moisture of the process air, which is carried along, isabsorbed.

[0005] Basically, several such drying cartridges are used, which areconnected in parallel. While at least one of the drying cartridges isoperating in the drying phase, the absorbed moisture is removed at thesame time from one or more drying cartridges, which are connected inparallel, so that these cartridges are prepared for a new drying phase.

[0006] It is, for example, known from the state of the art that theregeneration of the drying cartridges may be conducted according to theso-called co-current principle, according to which the direction of flowin the drying cartridges is the same in the drying phase for the processair and the regeneration phase of the drying cartridge.

[0007] In the case of the so-called counter-current principle, thedirection of flow during the regeneration of the drying cartridges isopposite to that during the drying process.

[0008] The advantage of the counter-current principle lies in thedistinctly lower energy consumption during the regeneration. While thedrying cartridge is absorbing moisture from the process air during thedrying process, it is moistened continuously in the direction of flow. Afront, separating the moist region from the dry cartridge, migrates inthe direction of flow. In good time, before this “front”, so formed andmigrating through the drying cartridge, has reached the end of thedrying cartridge, the flow of the process air must be switched over to adifferent drying cartridge while a safety or buffer zone is maintained.By these means, it is ensured that a dry zone is available withouttemporal interruption and that the drying process can, accordingly, takeplace continuously without interruption.

[0009] For the countercurrent regeneration, the still-remaining dryregion of the drying cartridge is now used as the starting point, fromwhich the adjoining, moistened regions of the drying cartridge are driedonce again continuously in a direction opposite to the precedingmoistening of the drying cartridge. The front, separating the moist fromthe dry region, now migrates back opposite to its original direction ofmovement.

[0010] In the case of the previously described co-current regeneration,the front, separating the moist from the dry region, rolls over theregion of the drying cartridge, which initially remains dry, andinitially still absorbs moisture from the region of the dryingcartridge, preceding the front separating the moist from a dry region.

[0011] Finally, the “by-pass method” is also used in some cases, forwhich the regeneration of the drying cartridge takes place at all timeswith a partial amount of the process air. For this purpose, 15 to 20% ofthe process air is diverted from the main stream, heated to about 220°to 300° C. and supplied to the regenerating drying cartridge. At the endof the drying process, the heating, assigned to the drying cartridge,which is to be regenerated, is switched off and the regenerated dryingcartridge is cooled to about 60° C. with the diverted stream of processair. Only when the temperature has been lowered to such a level, is themolecular sieve in the drying cartridge, which is to be regenerated,once again fully effective. At the same time, however, a constantlyvented partial amount of the process air must be taken up in the cycleas fresh air, so that the process air, to a certain extent, isadditionally loaded with moisture and the drying cartridge in the dryingprocess is saturated more quickly with moisture.

[0012] As is evident from the state of the art, there have been numerousefforts to improve the functionality and efficiency of the regeneration.

[0013] It is an object of the invention to create a regeneration processfor drying cartridges, which, with the least possible expenditure forequipment and a relatively low consumption of energy, makes possible anoptimized regeneration of the drying cartridges.

[0014] This objective is accomplished with the help of thedistinguishing features of claim 1.

[0015] Advantageous developments of the method arise out of thefollowing dependent claims.

[0016] An advantageous arrangement for carrying out the method is givenin claim 5 as well as the associated dependent claims.

[0017] The invention is explained in greater detail in the followingwith reference to the Figures, in which

[0018]FIG. 1 shows a diagrammatic representation of a device for dryingplastic granulate and for regenerating the process air, moistened duringthe drying process of the plastic granulate with the help of a molecularsieve,

[0019]FIG. 2 shows an adsorption isotherm of the molecular sieve and

[0020]FIG. 3 shows a graphic representation of the variation intemperature during the regeneration.

[0021] In FIG. 1, the region 10, which is at the right and has a border,shows a drying container 12, to the lower region of which process air,coming from a heater 11, is supplied. The process air flows through thegranulate, which is to be dried, and emerges once again from the upperregion of the drying container 12. The process air passes through afilter 13 and then reaches a blower 14, from which the process air ispassed on to the drying region 20.

[0022] At least two drying cartridges 24 a and 24 b, to which theprocess air, which is to be dried, can be supplied over the valves 22 aand 22 b, are disposed in this drying region 24 for the process air.

[0023] In a first phase, the process air, which is to be dried ordehumidified, is supplied over a valve 22 b to a drying cartridge 24 b.As the process air flows through the molecular sieve, disposed in thedrying cartridge 24 b, the moisture, contained in the process air, isabsorbed in the molecular sieve. In the molecular sieve, a moistenedinlet region and an adjoining dry region are formed.

[0024] At the same time, the front, separating the moist from the dryregion, migrates continuously from the inlet region to the outlet regionof the drying cartridge 24 b.

[0025] During the drying process in the drying cartridge 24 a, whichtakes place at the same time, hot atmospheric air is supplied in a firststep of the process by a blower 21 over a vow of 23 a and a heater 25 a.The fresh air, heated with the heater 25 a to a temperature of about220° to 300° C., is supplied to the regenerating drying cartridge 24 acountercurrently to the direction of flow of the process air, until thefront, which separates the dry from the moist region and moves from thebottom to the top, has reached the upper region of the drying cartridge24 a. When the front, separating the dry from the moist region, hasreached the upper region of the drying cartridge 24 a, the second stepof the process is initiated. The heater 25 a is switched off and thevalve 23 a is redirected in such a manner, that dry process air at atemperature of about 60° C. is passed into the drying cartridge 24 a,which is to be regenerated, for the purpose of follow-up drying andcooling. Moreover, this dry process air is introduced in a directionopposite to that established for the process air, which is to be dried.

[0026] For the processes, described above, it is important that, in goodtime, before the front separating the moist from the dry region of thedrying cartridge 24 b, employed for the drying process of the processair, has reached the outlet region of the cartridge, the valves 22 a, 22b, 23 a and 23 b are controlled, so that the process air, which is to bedried, is no longer supplied to the drying cartridge 24 b and, instead,is supplied to the previously regenerated, other drying cartridge 24 a,through which it now flows from top to bottom.

[0027] The processes, described above, proceed now with roles exchangedbetween drying cartridges 24 b and 24 a

[0028] Instead of the drying cartridges 24 a and 24 b, given in theexample, further, additional drying cartridges can also be provided. Byappropriately selecting the number of drying cartridges for the dryingand the regenerating cycle, it is possible to take into account thedifferent periods, during which the drying cartridges 24 a and 24 b areused for drying the process air on the one hand and regenerating thedrying cartridges 24 a and 24 b on the other.

[0029] Since the regenerating process of the drying cartridges 24 a and24 b takes place relatively rapidly in relation the drying process, thecooling phase of the regenerated drying cartridges, which follows theregeneration, can already be used for a drying function of theseregenerated drying cartridges, which are to be cooled, and, moreover,optionally in parallel to the other drying cartridge or cartridges.

[0030] After the drying cartridge 24 a or 24 b has been dried with hotair at 220° to 300° C., it must be cooled to a temperature of about 60°C., since the molecular sieve of the drying cartridges 24 a, 24 bregains its full effectiveness once again only at such a loweredtemperature.

[0031] A heat exchanger may be provided for the re-cooling prices. Theunused thermal energy from the regeneration of the drying cartridges 24a, 24 b can be transferred with such a heat exchanger to the stream ofair drying the material to be dried. However, this can lead to problemsparticularly in the case of material dried at low temperatures. It maybe necessary to decrease the capacity of the heat exchanger or even toprovide a refrigerator.

[0032] When atmospheric air is used partly or exclusively, the qualityof the regeneration, especially during the cooling phase, also dependson the humidity of the atmospheric air. This becomes clear from theadsorption isotherms shown in FIG. 2.

[0033] The process air is heated to about 220° to 300° C. and preferablyhowever 250° C., before it is passed through the drying cartridge 24 aor 24 b for the purpose of regenerating the cartridge. By these means,the forces binding the water to the molecular sieve in the dryingcartridge 24 a, 24 b are canceled. The water can be taken up by theregenerating air. Energy is consumed. In FIG. 2, the molecular sieve isin state A1. The time, at which the drying cartridge 24 a, 24 b as awhole no longer gives off any water, can be determined from thevariation in temperature at the discharge side of the drying cartridge.At this time, the temperature rises clearly more rapidly, as can be seenin FIG. 3.

[0034] In the case of the known method, the molecular sieve reaches thevalue A2 in the representation of FIG. 2. However, this state alsodepends on external climatic conditions. Since only testate A2 isreached by methods working exclusively with atmospheric air, the dewpoint of the drier is clearly higher than in the case of the inventivemethod, for which state B2 is reached. The cause of this is the residualmoisture in the molecular sieve.

[0035] A partial stream of air, which is diverted from the process air,is used to cool the drying cartridge 24 a, 24 b. With that, the resultof the regeneration depends only on the dew point of the process air.Since a constant dew point of the process air is aimed for and largelyachieved, the result of the regeneration is also practically constant.

[0036] The energy used for the regeneration is optimized by selectingthe countercurrent principle, which has already been explained above.

[0037] For the regeneration, atmospheric air is aspirated with theregeneration blower 21, supplied to the regeneration heating 25 a bysuitable settings of the valve 23 a and heated there to a temperature ofabout 220° to 300° C. The air, so heated, reaches the drying cartridge24 a, which is to be regenerated, where it absorbs moisture from thedrying cartridge 24 a. At the same time, use is made of the fact thatthe molecular sieve can absorb less water at higher temperatures. Theair, laden with water, is discharged to the atmosphere by a suitablesetting of valve of 22 a.

[0038] In the next step of the process, the regeneration heating 25 a isthen switched off and the flow of air controlled in such a manner, thatalready dried process air, having a temperature of about 50 to 60° C.,is used for the recooling of the drying cartridge 24 a.

[0039] Subsequent drying of the drying cartridge 24 a takes place inconjunction with the heat stored in the molecular sieve. The subsequentdrying is independent of external conditions of temperature andhumidity. The molecular sieve strives to reach a lower level withrespect to the water content. In FIG. 2, the molecular sieve is now instate B1 and, when all the heat is used up, in state B2.

[0040] The flow of air can be controlled with valves 22 a, 22 b, 23 a,23 b in such a manner, that the regenerated drying cartridge 24 a can beused once again for drying the process air. Alternately, the processwith the drying cartridge 24 b proceeds in a very similar manner.

1. Method for the countercurrent regeneration of moisture-laden dryingcartridges, characterized in that atmospheric air is heated to atemperature of about 220° to 300° C. and supplied to a drying cartridge(24 a, 24 b) for drying the latter countercurrently and the subsequentrecooling of the drying cartridge (24 a, 24 b) takes place with apartial stream, which has been diverted from the dried process air. 2.The method of claim 1, characterized in that the atmospheric air isheated to 260° C.
 3. The method of one of the claims 1 or 2,characterized in that the recooling is carried out with a partial streamof air having a temperature of about 50° to 60° C.
 4. The method of oneof the claims 1 to 3, characterized in that, after a relatively shortregeneration time, the regenerated drying cartridge can already be usedin the cooling phase once again for drying process air.
 5. Arrangementfor carrying out the method of one of the claims 1 to 4, characterizedin that drying cartridges (24 a, 24 b) are present, which are provided,on the one hand, with first three-way valves (22 a, 22 b), between whicha cross-connection is disposed and, on the other, with second three-wayvalves (23 a, 23 b), between which a cross-connection is disposed, thesecond three-way valves (23 a, 23 b) being connected indirectly over aheater (25 a, 25 b) with the associated drying cartridge (24 a, 24 b).6. The arrangement of claim 5, characterized in that a blower (21) ispresent, over which atmospheric air can be introduced into theconnection between the second three-way valves (23 a, 23 b).